C acid decarboxylases required for C photosynthesis are active in the mid-vein of the C species Arabidopsis thaliana, and are important in sugar and amino acid metabolism

Cells associated with veins of petioles of C(3) tobacco possess high activities of the decarboxylase enzymes required in C(4) photosynthesis. It is not clear whether this is the case in other C(3) species, nor whether these enzymes provide precursors for specific biosynthetic pathways. Here, we investigate the activity of C(4) acid decarboxylases in the mid-vein of Arabidopsis, identify regulatory regions sufficient for this activity, and determine the impact of removing individual isoforms of each protein on mid-vein metabolite profiles. This showed that radiolabelled malate and bicarbonate fed to the xylem stream were incorporated into soluble and insoluble material in the mid-vein of Arabidopsis leaves. Compared with the leaf lamina, mid-veins possessed high activities of NADP-dependent malic enzyme (NADP-ME), NAD-dependent malic enzyme (NAD-ME) and phosphoenolpyruvate carboxykinase (PEPCK). Transcripts derived from both NAD-ME, one PCK and two of the four NADP-ME genes were detectable in these veinal cells. The promoters of each decarboxylase gene were sufficient for expression in mid-veins. Analysis of insertional mutants revealed that cytosolic NADP-ME2 is responsible for 80% of NADP-ME activity in mid-veins. Removing individual decarboxylases affected the abundance of amino acids derived from pyruvate and phosphoenolpyruvate. Reducing cytosolic NADP-ME activity preferentially affected the sugar content, whereas abolishing NAD-ME affected both the amino acid and the glucosamine content of mid-veins.